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RT 244 -12 Wk 14 Digital Artifacts & Imaging ERRORS The advantages of CR are its large dynamic range, digital format, portability, and post-processing capability But not PERFECT –Imaging Errors can still occur See NOTES at bottom of slides for additional information Some information obtained from: AR Online » Current Issue » Artifacts and misadventures in digital radiography By Charles E. Willis, PhD, DABR; Stephen K. Thompson, MS, DABR; S. Jeff Shepard, MS, DABR Volume: 33 Number: 1 January 2004 1 Artifacts • Any irregularity on an image that is not caused by proper shadowing of tissue by the primary x-ray beam. • Are undesirable optical densities or blemishes on a radiograph. • Can be very interesting at times. You become the detective, what caused that? 2 • CR artifacts require special attention. • This is due to the fact that CR artifacts may be produced from various components of the CR system itself • Artifacts may also be generated by the users who are not aware of the proper imaging techniques or selection of appropriate image processing protocols • Since CR is also very sensitive to scattered radiation, it is vital that anti-scattered grids be used as in conventional radiography. • Radiographers should be concerned since these may generate unwanted artifacts that could not be corrected by any image processing algorithm. 3 Artifact Classification Review of Film/Screen 4 Artifact Classification Digital – CR & DR 5 OPTIMIZATION OF CR IMAGES Quality of CR images • Technologists are the key persons • delivering good quality radiographs • dose given to the patients. • CR images can NOT always be adjusted after exposing • CRITICAL to CR/DR Imaging is Technique, Positioning Collimation. 6 Optimization of a CR image quality may be achieved by optimizing the following factors • OBJECT ARTIFACTS: –Positioning and collimation • Exposure techniques • Image processing selection • Lifetime of the PSP 7 CR Artifacts • • • • Positioning errors Collimation errors Backscatter radiation LUT selection/histogram 8 Positioning of Part/ Collimation • 2 or more two projections on one IR • Is not a good practice with CR technique, • since double or multiple exposures on a single PSP) can lead to a failure of the image processing software to detect the image boundary. • Matching the positioning and collimation with the image processing parameters is also crucial. • Image processing will eventually fail to process since the input information is totally different. 9 10 Errors in collimation can cause mistakes in detection of the boundary, with a dramatic loss of image contrast 11 12 Positioning Errors 13 Collimation Errors 14 15 Loss of contrast due to partitioning errors 16 17 Alignment Error: Same technique, different centering and collimation S# 592 S# 664 18 19 2 on 24 X 30 Technique adjusted 2 on 24 X 30 Same technique Rescaling error. 20 Single field per plate - better 21 What is the problem? 22 Placement of gonadal shields is no longer trivial, but may adversely affect image quality. 23 Software to compensate for metal in patient 24 Same pt – what improved this image? 25 Acquiring good quality images • Regardless of the acquisition technology, good radiographic images can be produced only when certain fundamental requirements are met. • Appropriate radiographic technique must be used, • proper tube potential (kVp), • beam current (mAs), • source-to-image distance (SID), • collimation, • alignment of the X-ray central ray, • and positioning of detector and subject for the specific anatomic projection. 26 Exposure techniques • CR may be operated at a different film speed, and then optimizing the exposure technique accordingly. • Existing CR has a speed similar to medium speed film-screen system (200 -400) while spatial resolution is still generally inferior • Proper selection of an image processing algorithm specific to each type of x-ray examination is thus important. • The technical skills of radiographers definitely play a crucial role in determining the quality of the radiographic image. 27 Exposure factor “creep" • related to the wide exposure latitude of DR. • noise in DR images exposed at one-fourth to onehalf of the appropriate level. • artifacts are generally not apparent until the exposure exceeds 10 times the appropriate level. • Technologists avoid repeating an underexposed study by routinely increasing the radiographic technique. • Thus, the potential for gross overexposure exists in DR. Image optical density (OD), the usual indicator of proper exposure, is arbitrary in DR. • Managment of exposure factor in DR must rely on the value of a derived exposure indicator 28 Errors in the selection of the anatomic projection can cause inappropriate processing 29 Wrong Algorithm 30 Over/Under Exposed • Even though a CR image may be adjusted to improve the image visibility in the cases of over- or underexposures, it would still be impossible for an image processing to improve the visibility of clinical features that were not available in the raw image. 31 32 33 34 • Example of artifacts in CR an image with loss of contrast as a result of improper selection of image processing; • the same image now shows acceptable image quality as a result of proper selection of image processing. 35 36 37 Too many X-rays are a disservice to the patient and may also produce poor images (Figure 3). 38 Whether from underexposure or misalignment of a scatter reduction grid, too few X-rays produce noisy images 39 Different exam parameters in different rooms 40 Double exposure is a classic operator error • that constitutes approximately 2% of all rejected images. • The consequence of double exposure can be either a single repeated examination, when an inanimate object is involved (Figure 11), or two repeated examinations when two patients are involved (Figure 12). • In DR, double exposures can also be caused by power interruptions and communications errors, as well as by inadequate erasure secondary to overexposure or erasure mechanism failure. 41 Double exposure (Figure 11), 42 Double exposure # 12 43 44 #8 45 Edge Enhancement • The secondary function of image processing is to customize contrast in the region of interest • This type of image processing includes modifying the image to enhance the contrast and sharpness of some features while compromising the contrast and sharpness of others, • as well as modifying the image to make it appear more like a conventional film. • This secondary image processing is applied in a manner that is usually specific to the anatomic projection. 46 standard image edge sharpening 47 • Halo effect with Edge enhancement 48 49 Post Processing • An auxiliary purpose of image processing is to improve the usability of the digital image. • This includes imprinting demographic overlays, adding annotations, applying borders and shadow masks, flipping and rotating, increasing magnification, conjoining images for special examinations like scoliosis, and modifying the sequence of views. • This processing may require a separate QC workstation 50 51 IR Artifact - Digital • Pixel Failure = CR & DR plates should last for thousands of exposures. Interpolation can be used to fix defects in a small area. • Ghosting artifacts = exposure to environmental radiation or incomplete erasure. 52 background radiation 53 Image Receptor Artifacts • Debris on image receptor in DR can be confused with foreign bodies 54 Image Receptor Artifacts Line caused from dirt collected in a CR Reader 55 imaging plate was not fully erased before the chest examination was performed 56 Dirt on screens 57 #9 58 59 60 61 62 Lifetime of the PSP • One of the major advantages of CR is that the phosphor plate is reusable. • However, there are a number of factors that may affect the lifetime of an imaging plate. • The plates are subjected to normal wear and tear from scratches, scuffs, cracks, and contamination with dust and dirt, which may interfere with the production of a good image. • The establishment of a well-organized quality control program will play an important role in assessing the clinical quality of the imaging plate. This may easily be carried out by artifact assessment and uniformity evaluation across the plate. 63 Software Artifacts 64 Processing Errors 65 Digital Radiography Image Sampling • Image sampling, the plate is scanned, and the image’s location and its orientation are determined. The size of the signal is then determined, and a value is placed on each pixel. A histogram is generated from the image data. • The raw data used to form the histogram are compared with a “normal” histogram of the same body part by the computer. 66 The Nyquist Theorem • States that when sampling a signal (such as the conversion from an analog to a digital image), the sampling frequency must be greater than twice the bandwidth of the input signal so that the reconstruction of the original image will be nearly perfect. • At least twice the number of pixels needed to form the image must be sampled. If too few pixels are sampled, the result will be a lack of resolution. 67 Aliasing & Grid errors = Moiré • Spatial frequency is greater than the Nyquist frequency a wraparound image is produced. moiré effect • Stationary Grids: grid lines and the scanning laser are parallel moiré effect 68 Moiré effect 69 Software Artifacts • Image Compression – Used with teleradiology. Compression techniques “lossless” or “lossy” 70 Image Compression • Lossless compression: image can be reconstructed to be exactly the same as the original image. Compressed 10% or 50%. • Lossy compression: image is compressed 100:1. Used only when fine detail is not required. Not useful for medical imaging. 71 Image Compression 72 • Wider dynamic range means that technologists have to pay attention to exposure indicator values, instead of brightness and contrast. Without this attention, patient dose will escalate. If exposure indicator logs are available, they need to be evaluated. If they aren't, this will need to be done manually. 73 Misuses of image processing include • compensating for inappropriate radiographic technique, • compensating for poor calibration of acquisition and display devices, • and surreptitious deletion of nondiagnostic images. • Image processing to recover nondiagnostic images to prevent re-exposure should be a last resort, not a routine activity. • Routine reprocessing indicates a problem with automatic image processing or technical practice. Access to imageprocessing software is essential to develop and maintain appropriate processing parameters 74